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The invention pertains to an apparatus for eddy current testing of the conductance (or conductivity) or changes in the conductance (or conductivity) of a sample (such as a semiconductor wafer). The term xe2x80x9csamplexe2x80x9d is used throughout this specification to denote a specimen of which one intends to measure the electrical conductance.
For a variety of commercially significant purposes it is desirable to perform nondestructive tests to measure the electrical conductance, conductivity, resistance or resistivity of a sample. For example, during semiconductor product manufacturing, there is a need to measure the conductivity of various conductive thin films on semiconductor wafers and integrated circuits in a nondestructive manner.
It is well known that such measurements can be obtained by eddy current testing. One conventional apparatus for performing eddy current testing on a sample is described in U.S. Pat. No. 4,000,458 issued Dec. 28, 1976. The electrical conductivity of a lamella of conducting material (e.g., semiconductor wafers or metal films) is measured by introducing the lamella into the oscillatory magnetic field of the inductive element of the L-C tank circuit. The tank circuit is a portion of an oscillator circuit, whose driving current is adjusted, upon sample introduction, to restore the voltage amplitude of oscillation. The incremental current supplied to the tank circuit is linearly proportional to the sheet conductivity of the lamella.
Another is described in Jeanneret, et al., xe2x80x9cInductive Conductance Measurements in Two-Dimensional Superconducting Systems,xe2x80x9d Applied Phys. Letts. 55 (22), pp. 2336-2338 (Nov. 27, 1989). The Jeanneret, et al., apparatus employs two coils, both positioned above the sample: a drive coil, and a receiver coil. As the drive coil is driven by an AC voltage source (at a frequency of 70 kHz), the in-phase and quadrature components of the voltage at the receiver coil are measured by xe2x80x9cconventional lock-in techniquesxe2x80x9d or by an AC mutual-inductance bridge. The resulting voltage data can be processed (with data indicating the coils"" distance from the sample) to determine the sample""s complex conductance.
Several techniques for conventional eddy current testing are described in Semiconductor Measurements and Instrumentation, edited by W. R. Runyan, McGraw Book Company (1975), on pages 84-85. The sensitivities of conventional techniques, however, to sheet resistivities are limited to a few kxcexa9 and below.
We have developed an inventive apparatus for contactless measurement of sample conductances. The advantages of said apparatus, over other apparatuses described above, includes increased sensitivity, stability, and dynamic range, and decreased noise levels and thermal sensitivity. Said apparatus relies, in part, on the relative precision of RF amplitude detection. With said increased sensitivity of said apparatus, we are also not required to operate with a ferrite core (such as in Miller et al). Sufficient sensitivity for many applications is achieved with the use of xe2x80x9copen coils.xe2x80x9d The ability to use xe2x80x9copen coilsxe2x80x9d allows a greater flexibility and enhanced range of potential applications. In one embodiment of this invention the conductance measurement accuracy is enhanced by use of two RF amplitude detectors each producing DC signals. Thereafter the processing of only DC signals is required. Conventional contactless sample conductance measurement techniques have relied on more complex processing methods of RF signals that are inherently prone to electrical interference and noise. Said complex processing methods yield numerical values of sample sheet conductances that are less accurate than values achieved with this invention. One embodiment of the invention, detailed below, allows us to measure readily surface sheet conductances in a range from 100xcexa9xe2x88x921 to 10xe2x88x925xcexa9xe2x88x921.
The inventive apparatus is capable of accurate contactless sample conductance measurements. In accordance with the invention, a three coil apparatus for inductive conductance measurements comprises at least three coils, (or inductive devices,) a radio frequency (RF) generating device in conjunction with electronic circuitry for radio frequency amplitude measurement and comparison of radio frequency amplitude signals. The attainable accuracy is improved over that achieved using other conventional non-contact means by processing the differences of RF amplitude signals observed across pairs of sensing coils. Also, this invention does not require more complex RF signal processing, such as analysis of in-phase and quadrature voltage data. In a preferred embodiment, the natural resonance frequencies of the two sensing coils are tuned slightly off the RF operating frequency such that a monotonic response across a wide range of the sample""s conductivity is achieved. The sensitivity of the apparatus to low sample conductances is thereby further enhanced.